In the field of mobile communications so-called roaming allows a user to be reachable over different networks and thereby expand the service range beyond his home network. Roaming, i.e. the capability of a mobile terminal to not only access the home network of the user of the terminal but also other networks different from the home network, which are referred to as visited networks, has been possible mainly thanks to the proliferation of a specific technology all over the world, i.e. the Global System for Mobile communication (GSM), and standardized procedures for identifying and registering users in home and visited networks.
In recent years efforts have been made to provide communication services using networks based on the Internet Protocol (IP). An example of such a system is the so-called IP Multimedia Subsystem (IMS). In IMS networks roaming is also possible for terminals having subscription to an IMS network. If a terminal wants to register with a visited IMS network, it sends its own IMS identity that indicates has the form of a Uniform Resource Indicator (URI), i.e. points towards the home network as a domain for locating the home IMS subscription. The visited IMS network has a so-called Home Subscriber Server (HSS) for managing subscriptions of home subscribers and visiting subscribers that then contacts the home network, and provides service based on the IMS identity received from the visiting terminal.
The number of different telecommunication systems is increasing. These are not necessarily compatible with one another. Especially, even if a terminal has the technical capabilities of accessing different kinds of networks, e.g. is a “world” phone that supports different network accesses (e.g., GSM, UMTS, High Speed Packet Access (HSPA), and different radio frequency bands), and also may access to different telephony services such as circuit switched (CS) and packet switched (PS) services, then access to an IMS network still also requires the possession of a universally understandable IMS identity. However, this may not always be the case. For example, a subscriber of a pure GSM CS provider will only possess a Subscriber Identity Module (SIM) suitable for GSM or a module suitable for GSM and the Universal Mobile Telecommunications System (UMTS), which is referred to as a Universal SIM (USIM), and might only have a CS subscription with his home operator. There will then not be any IMS identity and hence no IMS subscription in a HSS, no “home” IMS network etc.